In general, as magnetic recording media for audio, video, computer (disks or memory tapes), etc., a magnetic recording medium having on a non-magnetic support a magnetic layer composed of a ferromagnetic fine powder dispersed in a binder has been used.
Recently, these magnetic recording media have been required for high density recording. Increase of S/N (signal/noise) and the reduction of noise have been attained by fining the size of a ferromagnetic fine powder, the employment of a ferromagnetic alloy powder, increased packed density of a ferromagnetic powder, and providing a very smooth surface of a magnetic recording medium.
Also, for performing high density recording without increasing time, it is necessary to quicken the recording (writing) speed to a magnetic recording medium or the reading speed from the magnetic recording medium and thus it has been required to run the magnetic recording medium at a high speed. For the high-speed running aptitude, the improvement of the running property, the antistatic characteristics, and the head cleaning characteristics of the magnetic recording medium must be considered and for such purposes, fillers such as carbon black and abrasives having a Moh's hardness of 8 or higher are used as disclosed in JP-A-59-193533, JP-A-59-186125, JP-A-59-191133, JP-A-58-189831, JP-A-59-63029, etc. (The term "JP-A" as used herein means an "unexamined published Japanese patent application").
Another problem of magnetic recording media is whether or not, they can stably store magnetic records over several tens of years.
The problem of the stable storage aptitude in a magnetic recording medium is mainly summarized into the following three points.
That is, the 1st point is to restrain the change of the magnetic characteristics of a ferromagnetic fine powder being used, that is, to restrain the reduction of the magnetic characteristics of a ferromagnetic alloy powder by rust or the reduction of the magnetic characteristics of a Co-containing .gamma.-iron oxide powder by the change of the distribution of Co and Fe.sup.++ in the iron oxide grains.
The 2nd point is to restrain the deterioration of a binder being used. It is known that a coated layer of a binder is generally deteriorated by ultraviolet rays, temperature, and humidity (see, for example, E. F. Cuddhy, IEEE Trans. Mag., Mag 16, 4, 558(July, 1980). It is also known that a vinyl chloride-vinyl acetate copolymer resin and a cellolose series resin slowly deteriorate as compared to a polyurethane resin and are each used in an amount of from 10 to 60% by weight of whole binders for a magnetic recording medium.
The 3rd point is to restrain the increase of drop out in a magnetic recording medium. A drop out of a magnetic recording medium is generally increased by an internal cause of the magnetic recording medium itself and an external cause by external dust.
The drop out by the internal cause is caused by the formation of foreign matter from a magnetic recording medium and these foreign matters cause not only drop out but also clogging of a magnetic head gap and causing spacing loss to make difficult the reproduction of the magnetically recorded signals.
Many causes are known for the formation of this foreign matter and they have been investigated. Examples of such foreign matters are certain kinds of fatty acids, certain kinds of fatty acid esters, oligomers from polymer supports, inorganic salts, etc.
It is considered that the drop out by the external cause is a spacing loss caused by attaching of dust particles (hereinafter "dust") existing in the air to the surface layer of a magnetic recording medium by electrostatic attraction. For avoiding the generation of static electricity on the surface of a magnetic recording medium, a method of reducing the surface electric resistance of a magnetic recording medium is known and for that purpose, it is known to apply carbon black or an antistatic agent to the inside of the layer(s) constituting the magnetic recording medium or onto the layer(s) thereof.
However, it is very difficult to effectively restrain the occurrence of drop out by the external cause of dust by these conventional techniques. The reason is considered to be that even if the electrostatic attraction of dust onto a magnetic recording medium is restrained, a magnetic recording medium exists in a dust-existing circumstance in the air and a magnetic recording medium has a high possibility of being exposed to dust. When a magnetic recording medium is exposed to a dust-existing circumstance during production, dust inevitably attaches to the magnetic recording medium during rolling or winding of the magnetic recording medium upon itself to cause drop out.
Various investigations have hitherto been made for reducing the occurrence of drop out. For example, JP-A-61-85620 proposes the formation of a backing layer containing a polyurethane resin, a vinylidenechloride-acrylonitrile copolymer (e.g., Saran.RTM.), and a polyisocyanate compound together with fine carbon black grains having a mean grain size of from 30 to 150 m.mu. and coarse carbon black grains having a mean grain size of from 160 to 600 m.mu. at a definite ratio as non-magnetic powders. By the aforesaid technique, the occurrences of the increase of the friction coefficient and scraping of the magnetic layer are inhibited, the occurrence of drop out caused by the aforesaid occurrences can be reduced and thus the S/N characteristics can be improved. However, even in such case, it has been found that a further improvement is required for inhibiting the occurrence of drop out under severe conditions of 200 passes.
Also, JP-A-58-91528, JP-A 59-58629, JP-A-60-66319, and JP-A-61-16022 propose the use of the magnetic layer or backing layer having a specific Young's modulus.
Furthermore, JP-A-58-91528 also discloses a method of improving the mechanical strength of a magnetic recording medium having a whole thickness of not more than 17 .mu.m by employing the magnetic layer and the backing layer each having a Young's modulus of at least 1200 kg/mm.sup.2 in the total of the longitudinal direction and the width direction. Also, in the examples thereof, the Young's modulus of the backing layer in the longitudinal direction is considerably high as from 650 to 850 kg/mm.sup.2 and in such a magnetic recording medium, it was very difficult to restrain the occurrence of drop out by dust as the external cause. That is, since the backing layer had a considerably high Young's modulus, when the backing layer was superposed on the magnetic layer during rolling or winding the magnetic recording medium, dust existed at the interface of the layers to cause drop out.
JP-A-59-58629 proposes a method of inhibiting scraping off a high elastic modulus backing layer and preventing dust falling from the backing layer by forming a subbing layer of an electric modulus of lower than 10.sup.9 dyn/cm.sup.2 between the support and the backing layer having an elastic modulus of over 10.sup.9 dyn/cm.sup.2. However, even in this technique, it was very difficult to restrain the occurrence of drop out by dust as the external cause.
JP-A-60-66319 describes that by forming a backing layer having a Young's modulus of from 200 to 1,500 kg/mm.sup.2 (preferably from 400 to 1,500 kg/mm.sup.2), the durability of the magnetic recording medium is increased owing to the increased toughness of the backing layer and also the occurrence of scraping of the backing layer is inhibited. However, even with that technique, it is very difficult to restrain the occurrence of drop out by dust as the external cause.
Also, JP-A-61-16022 discloses that the running inferiority of a magnetic recording medium caused by the transfer of the unevenness of the backing layer onto the magnetic layer is improved by employing the backing layer having a Young's modulus of less than 5.times.10.sup.9 dyn/cm.sup.2. However, even by that technique, it is very difficult to restrain the occurrence of drop out by dust as the external cause.
Accordingly, it has been strongly desired to develop a magnetic recording medium having an excellent storage stability aptitude with reduced drop out caused by dust, etc., as the external cause.